This invention relates to a device for controlling a pickup head position in a reproducing device for an optical type discs such as Compact Discs in the Compact Disc Digital Audio System and optical type video discs and, more particularly, to a device for controlling a pickup head position for a search operation for selection of a music piece.
A general construction of an optical head in an optical type disc reproducing device is shown in FIG. 1. The optical head is constructed of an optical system 1 and a magnetic circuit 2. The magnetic circuit 2 comprises a cylindrical center pole 3 formed in the center and a bobbin 4 provided about the center pole 3. An object lens 5 is provided on the outer end of the bobbin 4 and the bobbin 4 is supported by a suspension 6. On the bobbin 4 are wound a focus actuator (a vertically driving coil for focusing) 7 and a tracking actuator (a radially driving coil for tracking) 8. The object lens 5 is displaced by these actuators 7 and 8 in the vertical direction (i.e., direction normal to the disc surface) X and radial direction (i.e., radial direction of the disc) Y, and the focus of a laser beam projected through the cylindrical center pole 3 is controlled so that the laser beam is in focus at a specified track on the disc. The optical head is fed in the radial direction of the disc through a feed device composed of a feed motor 9, a reduction gear 10 and a feed gear 11.
An optical type audio disc reproducing device has a search function for searching designated musical piece etc. There have been the following pickup head position control devices for achieving the search function.
(1) A device of a type in which search is made by combination of oscillation (natural restoring force) of the suspensions 6 and on-off control of the tracking servo and employment of a forcible feed signal as disclosed in Japanese Patent Application No. 191434/1983. According to this device, a feed signal (a constant DC voltage) is applied to the feed motor 9 in a feed-servo-off state to forcibly displace the optical head toward a target position and during this period of time the tracking servo is periodically switched on and off to read position information at skipped locations on the disc and thereby achieve the search. When the tracking servo is off, the tracking actuator 8 is deviated toward the target position by the oscillation of the suspension 6 and the track is detected by switching on the tracking servo again at turning points of the oscillation at which the speed of oscillation becomes zero and reading of the disc position information is performed.
FIG. 2 schematically shows the movement of the tracking actuator 8 during this search. The tracking servo is on during the period i-ii and reading of the disc position information is performed. Since a forcible feed is performed during this period, the position of the track (i.e., position at which the laser beam is radiated) relative to the position of the optical head is gradually delayed so that the tracking actuator 8 is gradually deviated rearwardly. When the tracking servo is switched off at the point ii, the tracking actuator 8 is deviated toward the target position due to the oscillation of the suspension 6. At the turning point iii of the suspension 6, the tracking servo is switched on again to catch the track and effect reading of the disc position information during the period of iii-v. At the point v, the tracking servo is switched off and the suspension 6 is caused to effect the oscillation to deviate the tracking actuator 8. In this manner, the search of the target position is performed by setting the on-time and off-time of the tracking servo and reading the disc position information at skipped positions on the disc.
The reading device of the above described type is advantageous in that it can be constructed with low-price component parts, that the sequence control is easy to make as compared with a device of a type in which the position information is constantly read and that a quick search is available when the oscillation frequency of the suspension 6 is sufficiently high. On the other hand, the device of this type has the disadvantages that a quick search cannot be made when the oscillation frequency of the suspension 6 is low and that tolerance to irregularity of the oscillation frequency of the suspension 6 is relatively narrow, that is, when the off-time of the tracking servo is specifically determined, the position at which the tracking servo is switched on again does not necessarily coincide with the turning point of the oscillation of the suspension 6 (i.e., the position at which the oscillation speed becomes zero) if there is irregularity in the oscillation frequency of the suspension 6 resulting in failure in detecting the track.
(2) A device of a type in which a kick pulse is continuously given to effect the search.
According to the device of this type, the target position is detected by reading the disc position information while jumping the tracks by providing the kick pulse continuously. This is a modified form of fast-feed and fast-return functions of a conventional compact disc player of the Compact Disc Digital Audio System with increased speed.
The device of this type is advantageous in that the suspension 6 does not oscillate since the tracking servo is always on and, accordingly, the device is not affected adversely by the irregularity of the oscillation frequency of the suspension 6 (and other irregularities also) and that the cost of manufacture is reduced since the device does not require any special component parts for oscillation. The device, however, has the disadvantages that power consumption is relatively large since the kick pulses are continuously given, that a relatively heavy load acts on the tracking actuator 8 with a result that problems of generation of heat and reduction in reliability of the actuator 8 arise and that a quick search cannot be expected due to limitation in the allowable power in the tracking actuator 8.
(3) A device of a type in which a high-accuracy position detection and employment of the kick pulse are combined.
In contrast to the devices of (1) and (2) in which search is made while reading information during the movement of the optical head, the device of (3) utilizes a stepping motor as the feed motor 9 and does not effect reading of information during the movement of the optical head. According to this device, the stepping motor is driven by a certain amount depending upon difference between information of position before moving (i.e., present position) and the target position to bring the optical head to a position in close proximity of the target position while the tracking servo is off. Then the tracking servo is switched on and the tracking actuator 8 is driven by the kick pulse to search the target position.
This device is advantageous in that it has less element of instability than the devices of the other types and its operation is accurate because no reading of information is made during the movement of the optical head and that power consumption is relatively small since the kick pulse is given only in the last stage. The device, however, has the disadvantages that it requires a stepping motor which is quite expensive and that change in the target during the movement of the optical head is not possible or requires two operations (i.e., stopping and re-setting of the target position) because no reading of the position information is made during the movement of the optical head with a result that a quick search is not available.